1. Field of the Invention
The present invention relates to a control device and a control method, and a planar light source and a control method of the planar light source, and particularly to a control device and a control method, and a planar light source and a control method of the planar light source that are suitable for use in driving an LED (Light Emitting Diode).
2. Description of the Related Art
In related art, techniques for controlling an LED by current driving are widely used (see Japanese Patent Laid-Open No. 2005-310999, for example).
For example, an LED driving circuit 1 using a buck converter as shown in FIG. 1 controls power supplied to an LED. Such a buck converter is already commercialized, and is itself used widely without being limited to the driving of an LED.
The driving of an LED 11 in the LED driving circuit 1 shown in FIG. 1 is performed by the detection of a current value by a resistance 21 and PWM control by a current controlling PWM (Pulse Width Modulation) 31. LED power control for controlling the brightness of the LED 11, that is, the control of power supplied to the LED 11 when the current controlling PWM 31 is ON is performed by a brightness controlling PWM 32. Incidentally, it is needless to say that power control (brightness control) on the LED 11 can be performed by current control.
An OSC (oscillator) 33 generates a signal of fundamental frequency, and supplies the signal to the current controlling PWM 31 and the brightness controlling PWM 32.
The current controlling PWM 31 controls the turning on/off of a FET 12 on the basis of a result of comparison of the value of a voltage occurring across the resistance 21 with a reference voltage generated by a current control voltage supplying DAC (Current DAC (digital to analog converter)) 36, the result of comparison being supplied from a comparator (comp) 34.
FIG. 2 shows a common example of the waveform of gate current of the FET 12, that is, the switching operation of the FET 12 and the waveform of current flowing to the LED 11.
When the FET 12 is ON, that is, when the gate current of the FET 12 flows, a current I1 flows through the LED 11, the FET 12, and the resistance 21 from VDD1 via a reactance 13. A charge is accumulated in the reactance 13 during a period t1 when the FET 12 is ON. A capacitor 14 is for smoothing power.
The comparator (comp) 34 supplies the current controlling PWM 31 with a result of comparison of the value of voltage generated across the resistance 21 with the reference voltage generated by the current control voltage supplying DAC 36. The current controlling PWM 31 controls the FET 12 to turn on the FET 12 at the time of a start of one switching cycle. The current controlling PWM 31 maintains the ON state of the FET 12 while the value of the voltage generated across the resistance 21 is lower than the reference voltage generated by the current control voltage supplying DAC 36. The current controlling PWM 31 controls the FET 12 to turn off the FET 12 when the value of the voltage generated across the resistance 21 becomes higher than the reference voltage generated by the current control voltage supplying DAC 36.
In a period t2 in which the FET 12 is off, the charge accumulated in the reactance 13 is discharged, and a diode 15 acts to generate a current I2. Thus the current corresponding to the accumulated charge flows through the LED 11. Due to this series of operations, the current to the LED 11 is maintained in a state of equilibrium.
The brightness controlling PWM 32 determines an ON duty for PWM control on the basis of an n-bit PWM adjustment value obtained from an operating input unit not shown in the figure or an external device or the like by a PWM adjustment value obtaining unit 35, and outputs a signal for ON/OFF control of the FET 12.
Specifically, as shown by an LED current waveform of FIG. 3, when n-bit control (for example gradation control in 1024 steps when n=10) is performed, the brightness controlling PWM 32 outputs a signal for performing ON/OFF control of the FET 12 so that the ON operation of the FET 12 as described with reference to FIG. 2 is performed in predetermined steps. For example, the operation of turning on the FET 12 for the period t1 on the basis of a result of detection of a current value by the resistance 21 is repeated until an 800th step of the 10-bit gradation, that is, the 1024 steps, whereas the FET 12 is controlled to be off in the other 224 steps.
Specifically, the brightness controlling PWM 32 supplies an ON signal (1) to a logic circuit (logical sum) 37 for a period corresponding to 800 steps of the 1024 gradations, for example, as described above, that is, for a period of 800 steps of the 1024 steps of a PWM cycle, and supplies an OFF signal (0) to the logic circuit 37 for a period of the other 224 steps. The current controlling PWM 31 supplies an ON signal (1) to the logic circuit 37 for the period t1 in each step and supplies an OFF signal (0) for the remaining period t2 on the basis of the signal output from the comparator 34. The FET 12 is turned on when the current controlling PWM 31 and the brightness controlling PWM 32 both output the ON signals (1).